Signals from multiple users are multiplexed and transmitted to a base transceiver station. The multiplexing technique is also referred to as multiple access. As a method for multiple access, FDMA (Frequency Division Multiple Access) and OFDMA (Orthogonal Frequency Division Multiple Access) are well known. FDMA is a method of dividing the frequency range with a limited spectrum over a frequency axis, assigning the divided frequencies to respective users, and multiplexing the frequencies for transmission. OFDMA is translated as orthogonal frequency division multiplexing, and is a method of orthogonalizing the spectrums of adjacent channels with each other.
As known documents describing OFDMA, for example, there are Non-Patent Document 1 and Non-Patent Document 2. As known documents describing FDMA, for example, there are Non-Patent Document 3 and Non-Patent Document 4.
In each of OFDMA and FDMA, since multiple data streams are transmitted in parallel, each stream is transmitted over a single frequency channel. For this reason, in order to prevent the interference between the respective frequency channels (frequency bands), the configuration will be made as described below.
FIG. 20 shows frequency spectrums of OFDMA. FIG. 20 shows a case where there are six frequency channels in the system overall bandwidth W1, as an example of OFDMA frequency spectrums. According to OFDMA, multiple users (user 1 to user M) each transmit data by use of some of the multiple spectrums shown in FIG. 20. (FIG. 20 shows an example case where the user 1 uses two frequency channels, the user k uses a single frequency channel, and the user M uses three frequency channels.) In OFDMA, by overlapping adjacent frequency channels, the entire system bandwidth can be made small and the usage efficiency of the spectrum can be improved. However, to maintain orthogonality of adjacent frequency channels, constraints on the time synchronization of the multiplexed users are to be met. Non-Patent Document 1 and Non-Patent Document 2 describe that when the users cannot keep cooperation in transmission, the orthogonality of signals is destroyed and the characteristics of data transmission is significantly degraded.
FIG. 21 shows frequency spectrums of FDMA. FIG. 21 shows a case where there are six frequency channels in the system bandwidth W2, as an example of FDMA frequency spectrums. As described in Non-Patent Document 3, in FDMA, guard bands are provided between adjacent frequency channels to prevent the interference between the frequency channels to be used by the respective users. As a result, the entire system bandwidth of the FDMA system is greater than that of the OFDMA system, whereas the FDMA system has an advantage of eliminating the constraints on temporal synchronization among users. Nevertheless, the signal waveform that avoids the Inter-Symbol Interference in each frequency channel has to be employed for the FDMA system to maintain the signal quality in each channel. In order to avoid Inter-Symbol Interference at such a waveform level, the pulse waveforms of the signals are shaped based upon Nyquist criterion.
In FDMA, it is desirable to make smaller the intervals between the frequency channels, so that the frequency use efficiency is improved by making smaller the entire system bandwidth. For instance, FIG. 22 shows a case where the intervals between the frequency channels are made narrower by making the guard bands smaller. FIG. 23 shows a case where the intervals between the frequency channels are made narrower by making steep the pulse waveform of each frequency channel. Additionally, Non-Patent Document 4 describes a method of making narrower the bandwidth occupied by each frequency channel by employing Partial Response method that intentionally introduces Inter-Symbol Interference at the information symbol level.
Incidentally, the signal is shaped by a pulse shaping filter. However, when the signals are shaped by a filter with a sharp frequency response at the frequency domain as shown in FIG. 23, the time responses will be greatly dispersed at the time domain, making it difficult to design such a filter in practice. Accordingly, for shaping the pulses, a trade-off relationship exists between the shape in the frequency domain and the magnitude of the temporal dispersion of the shaped pulses in the time domain.
Non-Patent Document 1: S. B. Weinstem and P. M. Ebet, “Data transmission by frequency-division multiplexing using the discrete Fourier transform,” IEEE Trans. Commun., vol. 19, no. 5, pp. 628-634, Oct. 1971.
Non-Patent Document 2: Burton R. Saltzberg, “Performance of an efficient parallel data transmission system,” IEEE Trans. Commun., vol. 15, no. 6, pp. 805-811, Dec. 1967.
Non-Patent Document 3: J. G Proakis, “Digital Communications,” pp. 897-899.
Non-Patent Document 4: J. G Proakis, “Digital Communications,” pp. 561-568.